This invention relates to a water-soluble copolyester which is excellent in adhesion to a polyester film base and a hydrophilic colloid, particularly gelatin, and also improved in problems of working environment during use, and a polyester film having said copolyester as a subbing layer.
Polyester film, particularly biaxially oriented polyethylene terephthalate film has been used as base for photographic film, base for drawing, magnetic recording tape for its transparency, dimensional stability, and excellent mechanical properties, etc. In these uses, adhesion between the polyester film base and a surface layer material has been improved generally by providing a subbing layer therebetween. On the other hand, in the prior art, for improvement between a linear polyester and a metal foil, various plastics, particularly a hydrophilic resin such as gelatin, etc., it has been proposed to use a water-soluble or water-dispersible copolyester as the subbing layer for the problems in working environment and such advantage that no explosion preventive installation is required. As such an example, Japanese Patent Publication No. 40873/1972 discloses a copolymer using 8 mole % or more of an ester forming compound having metal sulfonate group based on all the acid components and 20 mole % or more of a polyethylene glycol based on all the glycol components for the purpose of dispersing water, but use of these as the subbing layer can be readily estimated to lower water resistance of said subbing layer, namely water resistance of adhesion. Also, in Japanese Patent Publication No. 5476/1981, a saturated linear aliphatic dicarboxylic acid with 4 to 8 methylene groups is used for the purpose of improving adhesion, but also in this case, water resistance of the subbing layer cannot be said to be satisfactory. Further, in Japanese Unexamined Patent Publication No. 88454/1981, description is made about use of a substantially water-insoluble aqueous dispersion for improvement of water resistance, but since this contains a water-soluble organic solvent in the dispersion, there remains the problems in working environment, etc. Japanese Unexamined Patent Publication No. 248231/1985 is almost similar to the above-mentioned Japanese Unexamined Patent Publicaton No. 88454/1981, but different therefrom primarily in that no organic solvent is contained in the final aqueous dispersion. However, since an organic solvent is used in the process of preparing an aqueous dispersion, in addition to the problems in working environment, the steps of preparing the aqueous dispersion become complicated and undesirable in practical application.
All of the subbing layers containing copolyesters as mentioned above can acquire adhesion to polyester film base with relative ease, but good adhesion to a hydrophilic colloid such as polyvinyl alcohol, gelatin, etc. could not be obtained because of poor water resistance.